JPH08162664A - Semiconductor photodetector - Google Patents

Abstract

PURPOSE: To improve high speed response characteristics of an APD by reducing a valence band discontinuity and decreasing the trap of holes in a superlattice multiplying layer. CONSTITUTION: An n-type InP buffer layer 2, an n<-> type superlattice avalanche multiplying layer 3 [In0.52 Al0.48 As (barrier layer)/InAs0.30 P0.70 (well layer)], a p--type wide gap field drop layer 4, a p<-> type Inlays light absorbing layer 5, and a p<+> type InP cap layer 6 are grown on an n<+> type InP substrate 1, an insulating protective film 7 is formed, and then a p-type side electrode 8 and an n-type side electrode 9 are formed. The type II superlattice avalanche multiplying layer lattice-matched to the substrate becomes In0.52 Al0.48 As (barrier layer)/InP (well layer). However, in this example, the part of the P of the well layer lattice-matched to the substrate is replaced with As to realize the multiplying layer in which valence band discontinuity is small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor photodetector, and more particularly to an avalanche multiplication type semiconductor photodetector used in optical communication systems.

[0002]

2. Description of the Related Art In order to construct a high-speed, large-capacity optical communication system, a high-speed, low-noise, high-sensitivity photodetector is indispensable. As such a light receiving element, a pin
Type light receiving element (Electronics Letters magazine, 1984
, Vol. 20, p. 653-654), and avalanche multiplication photodetector (hereinafter referred to as APD) (IEEE Electronic Device Letters, 19
1986, Volume 7, pp. 257-258) is known. In particular, the latter can utilize the internal gain effect and is excellent in high-speed response, so that the high gain bandwidth product (GB
The product) can be obtained, and for example, a GB product of 75 GHz is realized in the InP / InGaAs APD.

However, In, which is a multiplication layer here,
Since the ionization rate ratio of P is as small as about 2, there is a limit to high sensitivity for low noise. To further improve the characteristics, it was necessary to artificially increase the ionization ratio.

Therefore, F. Capasso
Et al. Used AlGaAs / G to accelerate electron ionization by using a multiplication layer with artificial band discontinuity in the conduction band.
It has been demonstrated that the aAs system improves the characteristics. Furthermore, Kagawa et al.
Applied Physics Letter Magazine, 1989, 55.
(10), pp. 993-959, InGaAs / In
It was confirmed that the ionization rate ratio was increased in the APD having sensitivity in the wavelength band for optical communication using the AlAs superlattice.

However, in the light-receiving element having this structure, a large number of band discontinuities exist in the superlattice avalanche multiplication layer, and especially the band discontinuity in the valence band is easily trapped by the energy barrier of the multiplied carriers. Trap the hole 2
A problem occurs that the frequency response of -3 GHz or higher is deteriorated.

As a measure against this point, Japanese Unexamined Patent Publication No.
There are element structures proposed in Japanese Patent No. 61174 and Japanese Patent Application Laid-Open No. 4-3595796. In the former, a part of the well layer of the multiplication layer is composed of a semiconductor strained layer. In this semiconductor strained layer, degeneracy of the band of the valence band is resolved to generate two bands, a light hole and a heavy hole, of which the light hole band approaches the valence band edge of the barrier layer. This suppresses the trapping of holes into the hetero barrier. Further, in the latter APD, tensile stress is applied to the well layer of the multiplication layer. As a result, the mass of holes in the well layer becomes lighter than that in the case of bulk, and the pile-up phenomenon is alleviated.

[0007]

DISCLOSURE OF THE INVENTION Conventional typical APD
However, since holes are trapped in band discontinuities in the valence band that are present in a large number in the superlattice avalanche multiplication layer, there is a problem that high frequency characteristics are deteriorated. The device structures according to the above publications proposed to solve this problem all attempt to suppress the trapping of holes by introducing strain into the well layer, and therefore have the following drawbacks.

The strain introduced in the well layer eliminates the degeneracy of the band of the valence band and produces two bands, a band of light holes and a band of heavy holes. If the strain is compressible, the band of light holes approaches the edge of the valence band of the barrier layer, and if the strain is tensile, the band of heavy holes approaches. But,
Holes are still trapped because the band discontinuity has not been resolved. Moreover, if the strain is compressible, the band of heavy holes is pulled, and if the strain is tensile, the band of light holes is moved away from the valence band edge of the barrier layer. It is not possible to expect a sufficient effect of improving the high-frequency characteristics with the solution.

The present invention has been made in view of the above problems of the prior art, and its object is to inhibit the barrier layer and the well layer by changing the composition of the V group element of the superlattice structure avalanche multiplication layer. The difference in band width is mainly reflected in the band discontinuity in the conduction band, and the band discontinuity in the valence band is minimized to improve the ionization ratio and simultaneously achieve low noise, high multiplication, and high-speed response. To provide APD to do.

[0010]

To achieve the above object, according to the present invention, a superlattice structure avalanche multiplication layer made of a III-V group semiconductor and a light absorption layer are provided on a semiconductor substrate. A barrier layer and a well layer constituting the superlattice structure avalanche multiplication layer is a semiconductor having an energy band structure of a type II superlattice in a composition lattice-matched with the semiconductor substrate, and the barrier layer and And / or a semiconductor light receiving element characterized in that the composition of the group V element constituting the well layer is changed to reduce the amount of valence band discontinuity.

Further, according to the present invention, in the above-described semiconductor light receiving element, the composition of the group V element is changed so that the difference between the lattice constant of the barrier layer or the well layer and the lattice constant of the semiconductor substrate is different. When this occurs, a difference in the direction opposite to this difference occurs between the lattice constant of the well layer or the barrier layer and the lattice constant of the semiconductor substrate, and the group III element forming the well layer or the barrier layer. There is provided a semiconductor light receiving device, wherein the composition is changed to introduce an amount of strain into the well layer or the barrier layer to compensate the strain generated in the barrier layer or the well layer.

[0012]

1 (a) and 1 (b) are views for explaining the band structure of a semiconductor light receiving element according to the present invention, which shows one of a barrier layer and a well layer constituting a superlattice avalanche multiplication layer. A band structure for a period is schematically shown. FIG.
(A) is a band structure diagram when a semiconductor having an energy band structure that becomes a type II superlattice in a composition lattice-matched with a semiconductor substrate is used for a barrier layer and a well layer. FIG. 1B is a band structure diagram in the case where the composition of the group V element forming the well layer is changed to reduce the valence band discontinuity, and the band structure of the first embodiment of the present invention. Equivalent to. Further, FIG. 1C is a band structure diagram of a barrier layer and a well layer of a typical conventional superlattice structure (type I superlattice structure) avalanche multiplication layer.

In the conventional structure shown in FIG. 1C, In _0.52 Al _0.48 As / In is a material system in which the discontinuity amount of the valence band edge of the barrier layer and the well layer is sensitive to a wavelength of 1 to 1.6 μm.
_{When 0.53} Ga _0.47 As is used, it is 0.2 eV, and when holes generated by avalanche multiplication travel in a superlattice with many energy barriers, they are trapped by this barrier and the response speed becomes about 3 GHz or less. to degrade.

On the other hand, a type II composition with a lattice-matched substrate
When a semiconductor having an energy band structure that becomes a superlattice is used for the barrier layer and the well layer, the conduction band discontinuity and the valence band discontinuity shown in FIG. According to the invention, here, for example, the composition of the group V element is changed in the well layer. Generally, in a III-V group compound semiconductor, the value of the valence band edge energy is mainly determined by the V group atom.
Therefore, the value of the valence band edge energy can be adjusted by changing the composition of the V group element.

Specifically, part of the group V element composition of the well layer at the time of lattice matching is replaced with an element having a larger mass number. For example, when a type II superlattice composed of InAlAs and InP is used for the multiplication layer, the band structure shown in FIG. 1A is obtained, and the valence band discontinuity is 0.3 eV. here,
For InP in the well layer, a part of P of the V group element is replaced with As having a larger mass number than P. Figure 1 (b) shows 30%
7 is a band diagram in which P is replaced with As, and the valence band discontinuity is within 0.1 eV. In this case, the conduction band discontinuity expands to 0.5 eV, reflecting the decrease in the forbidden band width due to the addition of As. Thus, the valence band discontinuity can be made extremely small and the conduction band discontinuity can be made large, so that holes trapped by the band discontinuity are reduced and the high speed response is improved. It is possible to improve the ionization ratio.

On the contrary, after forming the multiplication layer of the type II superlattice band structure lattice-matched to the substrate, the composition of the group V element on the barrier layer side is changed (in this case, part of the group V element is It is also possible to reduce the discontinuity of the valence band by replacing the element with a smaller mass number), and in this case as well, it is possible to improve the ionization rate and the high-speed response at the same time as in the above case. .

When part of the group V element of the well layer (barrier layer) lattice-matched with the substrate is replaced with an element having a larger (smaller) mass number, the lattice constant becomes larger (smaller) than that of the substrate. Therefore, the well layer (barrier layer) receives compressive strain (tensile strain), and the entire multiplication layer also receives compressive strain (tensile strain).

In the present invention, in order to relax this strain, the composition of the group III element constituting the barrier layer (well layer) is adjusted so that the lattice constant of the barrier layer (well layer) is smaller than that of the substrate. Well layer (barrier layer) so that it becomes smaller (larger)
The strain for compensating for the strain generated in 1) is intentionally introduced into the barrier layer (well layer). Specifically, part of the group III element composition of the barrier layer (well layer) that was lattice-matched with the substrate is replaced with an element having a smaller (larger) mass number. As a result, the strain of the entire multiplication layer is relaxed, and the crystalline quality of the multiplication layer can be improved.

[0019]

Embodiments of the present invention will now be described with reference to the drawings. [First Embodiment] FIG. 2 is a sectional view of an avalanche multiplication type semiconductor light receiving device according to the first embodiment of the present invention. This light receiving element is manufactured as follows. That is, the following layers are sequentially grown on the n ⁺ type InP substrate 1 using the gas source MBE device. n-type InP buffer layer 2: Layer thickness: 1 μm n ^- type superlattice avalanche multiplication layer 3: Composition: In _0.52 Al _0.48 As (barrier layer) / InAs _0.30
P _0.70 (well layer) Carrier concentration: 2E15 cm ⁻³ Layer thickness: 0.3 μm (15 cycles of 10 nm each) p-type wide-gap electric field drop layer 4: Composition: InAlAs Carrier concentration: 8E17 cm ⁻³ , Layer thickness: 0. 1 μm p ⁻ type InGaAs light absorption layer 5 carrier concentration: 1E15 cm ⁻³ , layer thickness: 1.3 μm p ⁺ type InP cap layer 6 carrier concentration: 5E18 cm ⁻³ , layer thickness: 0.2 μm

Next, a circular mesa having a diameter of 50 μm is formed by a normal photolithography technique and wet etching to form an insulating protective film 7. Finally, p ⁺ type InP
A p-side electrode 8 and an n-side electrode 9 which are in ohmic contact with the cap layer 6 and the n ⁺ type InP substrate 1 are formed.

In the type II superlattice avalanche multiplication layer lattice-matched to the InP substrate, In _0.52 Al _0.48 As (barrier layer) / InP (well layer) is obtained. % As As, and the superlattice is made of In _0.52 Al _0.48 As (barrier layer 3a) / InAs.
_0.30 P _0.70 (well layer 3b), thereby realizing a multiplication layer with small valence band discontinuity and large conduction band discontinuity.

[Second Embodiment] FIG. 3 is a sectional view showing a second embodiment of the present invention. In the figure, it omitted the duplicated description because it is face down with the same reference numerals portion equivalent to parts of the first embodiment shown in FIG. 2, in this embodiment, n ^- -type The structure of the superlattice avalanche multiplication layer is configured as follows. Composition: In _0.52 Al _0.48 As _0.8 P _0.2 (barrier layer) / I
nP (well layer) Carrier concentration: 2E15 cm ^-3 Layer thickness: 0.3 μm (15 cycles of 10 nm each)

In contrast to the type II superlattice In _0.52 Al _0.48 As (barrier layer) / InP (well layer) lattice-matched to the InP substrate, 20% of As in the barrier layer is replaced with P in this embodiment, The superlattice is In _0.52 Al _0.48 As _0.8
Since P _0.2 (barrier layer 3c) / InP (well layer 3d) is used, a multiplication layer having a small valence band discontinuity and a large conduction band discontinuity is realized.

[Third Embodiment] FIG. 4 is a sectional view showing a third embodiment of the present invention. In the present embodiment, n ⁻
The type superlattice avalanche multiplication layer 3 is configured as follows. Composition: In _0.45 Al _0.55 As (barrier layer) / InAs _0.30
P _0.70 (well layer) Carrier concentration: 2E15 cm ^-3 Layer thickness: 0.3 μm (15 cycles of 10 nm each)

In the first embodiment, the well layer has a lattice constant of I.
Since InAs _0.30 P _0.70 larger than nP is set, strain is introduced into the multiplication layer. In this embodiment, the substrate (I
nP) is lattice-matched to In _0.52 Al _0.48 As, the composition of the group III element is changed so as to increase the ratio of elements having a small mass number, and In _0.45 A having a smaller lattice constant is used.
The _0.55 As barrier layer 3e compensates for the strain introduced by the InAs _0.30 P _0.70 well layer 3b. As a result, strain in the multiplication layer is relaxed, dislocations on the crystal are hard to enter, and high-quality epitaxial growth is possible.

[Fourth Embodiment] FIG. 5 is a sectional view showing a fourth embodiment of the present invention. In the present embodiment, n ⁻
The type superlattice avalanche multiplication layer 3 is configured as follows. Composition: In _0.52 Al _0.48 As _0.8 P _0.2 (barrier layer) / I
nAsP (well layer) Carrier concentration: 2E15 cm ^-3 Layer thickness: 0.3 μm (15 cycles of 10 nm each)

In the second embodiment, the barrier layer has a lattice constant of I.
Since In _0.52 Al _0.48 As _0.8 P _0.2 is smaller than nP, strain is introduced into the multiplication layer. In the present embodiment, a part of P in the InP well layer is replaced with another V group element so that InA having a lattice constant larger than InP.
As the sP well layer 3f, In _0.52 Al _0.48 As _0.8 P
_0.2 compensates for the strain introduced by the barrier layer 3c.
As a result, strain in the multiplication layer is relaxed, dislocations on the crystal are hard to enter, and high-quality epitaxial growth is possible.

In any of the above embodiments, the valence band discontinuity can be minimized and the conduction band discontinuity can be increased, so that the ionization ratio is increased while suppressing the trapping of holes. Can be low noise, high multiplication,
It becomes possible to realize an APD that simultaneously satisfies high-speed response characteristics.

The first and second embodiments can be used together. That is, a semiconductor having an energy band structure of a type II superlattice having a composition lattice-matched with a semiconductor substrate is selected for the barrier layer and the well layer, and then the composition of the group V element of both the barrier layer and the well layer is changed to change the valence. The same effect can be obtained even when the electron band discontinuity is reduced.

[0030]

As described above, in the semiconductor light receiving element of the present invention, the difference in the forbidden band width between the barrier layer and the well layer in the multiplication layer is mainly reflected in the band discontinuity of the conduction band, and the valence band Since the amount of band discontinuity is made as small as possible, it is possible to prevent degradation of response characteristics by keeping the ionization ratio high and reducing the number of holes trapped in the band discontinuity. Therefore, according to the present invention, low noise and high sensitivity
It is possible to realize an APD having a high-speed response characteristic, that is, a high GB product.

[Brief description of drawings]

FIG. 1 is an explanatory view of a band structure of a light receiving element of the present invention and a conventional light receiving element for explaining the operation of the present invention.

FIG. 2 is a sectional view of the first embodiment of the present invention.

FIG. 3 is a sectional view of a second embodiment of the present invention.

FIG. 4 is a sectional view of a third embodiment of the present invention.

FIG. 5 is a sectional view of a fourth embodiment of the present invention.

[Explanation of symbols]

1 n ⁺ type InP substrate 2 n type InP buffer layer 3 n ⁻ type superlattice avalanche multiplication layer 3a In _0.52 Al _0.48 As barrier layer 3b InAs _0.3 P _0.7 well layer 3c In _0.52 Al _0.48 As _0.8 P _0.2 barrier layer 3d InP Well layer 3e In _0.45 Al _0.55 As barrier layer 3f InAsP well layer 4 p-type wide-gap field drop layer 5 p ⁻ type InGaAs light absorption layer 6 p ⁺ type InP cap layer 7 insulating protection film 8 p-side electrode 9 n-side electrode

Claims (3)

[Claims] 1. A semiconductor light-receiving element comprising a superlattice structure avalanche multiplication layer made of a III-V group semiconductor and a light absorption layer on a semiconductor substrate, wherein a barrier layer and a well constituting the superlattice structure avalanche multiplication layer. The layer is a semiconductor having an energy band structure that is a type II superlattice in a composition that lattice-matches the semiconductor substrate, and the composition of the group V element forming the barrier layer and / or the well layer is changed to change the valence. A semiconductor light-receiving element characterized by reducing the amount of discontinuity in the electron band. 2. When a difference occurs between the lattice constant of the barrier layer or the well layer and the lattice constant of the semiconductor substrate due to the change in the composition of the group V element, the difference in the opposite direction to this difference is generated. The composition of the group III element constituting the well layer or the barrier layer is changed so that it occurs between the lattice constant of the well layer or the barrier layer and the lattice constant of the semiconductor substrate. 2. The semiconductor light receiving element according to claim 1, wherein an amount of strain that compensates for the generated strain is introduced into the well layer or the barrier layer. 3. The barrier layer and the well layer lattice-matched with a semiconductor substrate are InAlAs and InP, respectively, and V
2. The semiconductor light receiving element according to claim 1, wherein changing the composition of the group element is replacing part of As in the barrier layer with P and replacing part of P in the well layer with As.